Optical element made of fluoride crystal

ABSTRACT

A method for manufacturing fluoride crystal includes the steps of adding scavenger and a material to a crucible, melting the scavenger and material at a temperature higher than a melting point so that a ratio of a thickness of the fluoride crystal that has been melted to an inner diameter of the crucible may be 0.2 or higher, and gradually crystallizing and purifying the material.

BACKGROUND OF THE INVENTION

The present invention relates generally to crystal manufacture methodsand apparatuses, and more particularly to a manufacture method forcalcium fluoride (CaF₂) crystal usable as a material for opticalelements used for an exposure apparatus for photolithography.

Along with the recent demands on smaller and thinner-profile electronicdevices, finer semiconductor devices to be mounted onto these electronicdevices have been increasingly demanded. For example, a design rule fora mask pattern tries to achieve a line and space (L & S) of 130 nm on amass production line, and predictably it will be increasingly smaller inthe future. L & S denotes an image projected to a wafer in exposure withequal line and space widths, and serves as an index of exposureresolution.

The exposure has three important parameters, i.e., resolution, overlayaccuracy and throughput. The resolution is a minimum size for a precisepattern transfer. The overlay accuracy is a precision with which tooverlay multiple patterns over an object to be exposed. The throughputis the number of sheets exposed per unit of time.

A shortened wavelength of exposure light and a larger aperture of aprojection lens are effective for improved resolution. For the shortenedwavelength, a light source has been in transition from KrF excimer laser(with a wavelength of approximately 248 nm) to ArF excimer laser (with awavelength of approximately 193 nm), and F₂ excimer laser (with awavelength of approximately 157 nm) is about to reduce to practice. Onthe other hand, a lens with a diameter of 250 mm or larger has beendeveloped for the larger aperture.

The CaF₂ crystal has higher light transmittance (or internaltransmittance) for this wavelength range than other glass materials, andthus is the best as an optical material for optical elements, such as alens and a diffraction grating, applicable to an exposure opticalsystem.

Parameters for evaluating optical materials for a lens, etc. include, inaddition to the internal transmittance, laser durability indicative oftransmittance changes in response to continuous receptions of a laserbeam, refractive index homogeneity indicative of a degree of constant ofa lens's refractive index according to positions, birefringence, processor (grinding) accuracy, etc.

A method for manufacturing CaF₂ crystal with good one or more opticalcharacteristics among them has been already disclosed in JapaneseLaid-Open Patent Applications Nos. 9-315893 and 10-330192.

However, the CaF₂ crystal manufactured by the conventional manufacturemethods exhibit satisfactory optical characteristics for visible light,but has a disadvantage in that it has not yet exhibited satisfactoryoptical characteristics for short-wavelength and high-power light suchas excimer laser, e.g., low laser durability, large refractive indexhomogeneity and birefringence, poor surface precision at the time ofgrinding process, etc.

As a result that the instant inventor has eagerly studied the causes, hehas found that it results from the crystal or lattice defect or drossinclusion. The dross inclusion is generally removed through segregationfrom one-way cooling and crystallization in the purification process.

However, the crystal for use with the excimer laser has such a largeaperture that it has a difficulty in one-way crystallization and cannotsufficiently remove impurities disadvantageously.

This is because as the crystal aperture becomes large, a ratio ofthickness to aperture diameter becomes small, causing the uniform melttemperature distribution during melting, or the small temperaturedifference between top and bottom surfaces in the melt, as shown in FIG.4. Although FIG. 4 shows a temperature distribution with a ratio ofthickness to aperture diameter of 0.15, the dimension in FIG. 4 does notaccord with the description for simplicity purposes.

The temperature at the bottom surface of a skull crucible containing themelt is adapted to be lower than that at the top surface. Thepurification process is implemented at considerably higher temperaturethan the melting point due to dehydration and degas, and does not use aseed that is usually used for monocrystal growth.

Therefore, impurities of relatively light specific gravity gathering atthe top surface in the melt becomes a starting point for crystallizationor nucleus, resulting in crystallization from the top surface as well asfrom the bottom surface in the melt at a low temperature. As a result,impurities, such as rare earth, and bubbles concentrate in the middle ofcrystal, which is a part finally solidified, and are hard to be removed.

Table 1 shows impurity data in CaF₂ crystal, which is magnesium as anexample. It is understood that the middle has higher magnesiumconcentration than the top and the bottom. In other words, magnesiumaccumulates in the center. TABLE 1 Mg CONTENT TOP PART 0.55 MIDDLE PART1.00 BOTTOM PART 0.68

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an exemplary object of the present invention toprovide a novel and useful calcium fluoride crystal, its manufacturemethod, an optical element using the calcium fluoride crystal, anexposure apparatus using the optical element for its optical system, adevice fabrication method using the exposure apparatus, and a devicemanufactured by use of the exposure apparatus.

Another specific and exemplary object of the present invention is toprovide calcium fluoride crystal having excellent opticalcharacteristics including the laser durability, and its manufacturingmethod.

Another exemplary object of the present invention is to provide anoptical element using the calcium fluoride crystal, and an exposureapparatus using this optical element for its optical system, especiallyfor lithography using excimer laser as an exposure light source).

Still another exemplary object of the present invention is to provide adevice manufacture method using the exposure apparatus and high qualitydevices (such as semiconductors, LCDs, CCDs, thin magnetic heads)fabricated using the exposure apparatus.

A method for manufacturing fluoride crystal includes the steps of addingscavenger and a material to a crucible, melting the scavenger andmaterial at a temperature higher than a melting point so that a ratio ofa thickness of the fluoride crystal that has been melted to an innerdiameter of the crucible may be 0.2 or higher, and graduallycrystallizing and purifying the material. The inner diameter of thecrucible may be 250 mm or larger. The crucible is divided into aplurality of parts like a multistage structure. The method may includethe steps of melting the purified fluoride crystal, and growing thefluoride crystal to obtain fluoride monocrystal. The fluoride crystalis, for example, calcium fluoride.

An optical element made of fluoride crystal manufactured by the abovemethod, an exposure apparatus including an optical system that uses theabove optical element, and a device fabrication method including thesteps of exposing an object using the above exposure apparatus, andperforming a predetermined process for the exposed object constituteother aspects of the present invention.

The above optical element may include, for example, a lens, adiffraction grating, an optical film, or a combination thereof. Forexample, it may include a lens, a multi-lens, a lens array, a lenticulelens, a fly-eye lens, an aspheric lens, a diffraction grating, a binaryoptics element or a combination thereof. The optical element includes,for example, an optical sensor in addition to a single lens (e.g., foruse with focus control). The exposure light may use ultraviolet light,far ultraviolet light, and vacuum ultraviolet light as exposure light.

Claims for a device fabricating method for performing operations similarto that of the above exposure apparatus cover devices as intermediateand final products. Such devices include semiconductor chips like an LSIand VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, andthe like.

Other objects and further features of the present invention will becomereadily apparent from the following description of the preferredembodiments with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a purification furnace used forpurification.

FIG. 2 is a schematic sectional view of a purification furnace used forpurification.

FIG. 3 is a temperature distribution in a crucible according to aninventive manufacture method.

FIG. 4 is a temperature distribution in a crucible according to aconventional manufacture method.

FIG. 5 is a schematic sectional view of a growth furnace used forcrystal growth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for manufacturing fluoride crystal as one embodiment includesthe steps of purifying a calcium fluoride (CaF₂) material by filling andmelting the material so that a ratio of a thickness of melted fluoridecrystal to an inner diameter of a crucible may be 0.2 or higher.Following the purification, there is provided a step of removing ahazardous material for high purification, such a step for removingimpurities that has partially concentrated through segregation, such asrare earth using a grinder and a blaster. An optical element of oneembodiment is made of this CaF₂ crystal. Such calcium fluoride crystalincludes little impurities and gas, and thus would provide an opticalelement having excellent optical characteristics including refractiveindex homogeneity, birefringence, internal transmittance, and laserdurability.

A purification process adjusts a ratio of a thickness of melted crystaland an inner diameter of a crucible to be 0.2 or larger to avoidsolidification of a material from both the top and bottom. For example,the inner diameter in the crucible is dimensioned to be 250 mm orlarger. The amount of the material and the depth in the crucible aredesigned based on the bulk specific gravity of the material so that whenthe powder material is supplied and melted, a distance from the bottomof the crucible to the top surface of the melt is at least 0.2 times aslarge as the inner diameter in the crucible.

FIG. 3 shows a temperature distribution in the crucible when thematerial is melted in this way. Due to the large temperature differencebetween the top and bottom surfaces, the solidification occurs in onedirection, i.e., from only a surface at a lower temperature. As aresult, the top surface of the purified crystal is solidified finallyand air bubbles and impurities of small specific gravity accumulate onthe top surface while impurities, such as rare earth, accumulate on thetop and bottom surfaces in the crystal due to segregation, they areextremely removed in the post-treatment.

A concrete description will now be given of CaF₂ crystal using acrucible descent process of the preferred embodiment according to thepresent invention, although the present invention is not limited to thecrucible descent process.

Scavenger is added to and fully mixed with the fluoride material. Thesolid scavenger is added by 0.02 mol % to 2 mol %. The fluoride used asscavenger preferably includes zinc fluoride, cadmium fluoride, manganesedifluoride, bismuth fluoride, sodium fluoride, lithium fluoride, andthose which are more easily combinable with oxygen than the fluoride tobe grown, and easily decompose and evaporate.

The fluoride material added and mixed with scavenger is put in thecrucible of a purification furnace shown in FIG. 1. In FIG. 1, 1 is achamber of the purification furnace, which is connected to a vacuumpumping system. 2 is a heat insulation member, 3 is a heater, whichforms, in a space that accommodates the crucible inside the heater, atemperature distribution with a temperature range including a meltingpoint of the crystalline material, when the crystalline material thathas once melted gradually solidifies. 4 is a multistage crucible havingan inner diameter of 250 mm or larger and a depth of at least 0.6 timesas large as the inner diameter. 5 is calcium fluoride (CaF₂). 6 is a barthat supports the crucible, and is connected to a mechanism for movingthe bar up and down.

The furnace is vacuum-exhausted, and then the heater is electrified toheat the crucible. The furnace is initially heated at about 150 ° C. to200 ° C., whereby water and inert gas are eliminated. The furnace isthen heated so as to form the predetermined distribution of temperatureas mentioned above. The crucible is placed at a high temperature regionof the distribution, and then the CaF₂ material is heated via atemperature range 350° C.-1100° C. in which the scavenger is melted andthe scavenge reaction begins. This heating is set enough slow for thescavenge reaction.

After the material is melted, the melt condition is left for severalhours to tens of hours so as to eliminate residual impurities. Then, thecrucible is descended in the temperature distribution formed in thefurnace and the melted CaF₂ material is gradually cooled for crystalgrowth. The height of the melted material is set at least 0.2 times aslarge as the inner diameter of the crucible. A top of the crucible isthus maintained at a sufficiently higher temperature than the meltingpoint of CaF₂ in comparison with a bottom of the crucible, and a startof solidification is prevented at this part. This process does notrequire such precise temperature control as in the following process ofsingle crystal growth, and the obtained crystal may be multicrystal orinclude grain boundary.

Among thus obtained crystal, the top and bottom parts, i.e., first andlast crystallized parts are eliminated. Impurities concentrate orsegregate on these parts, and thus these parts are eliminated toeliminate impurities that negatively affect characteristics of thecrystal.

Single crystal is grown from the purified crystal as a material. Agrowth process is pursuant to the Bridgman method as an example.

The purified crystal is put in the crucible in the growth furnace shownin FIG. 5. In FIG. 5, 11 is a chamber for the growth furnace, which isconnected to a vacuum pumping system. 12 is a heat insulation member. 13is a heater. 18 is a crucible having an inner diameter of 250 mm orlarger. 15 is fluoride produced in the purification process. 16 is a barthat supports the crucible, and is connected to a mechanism for movingthe bar up and down.

The fluoride crystal is set in the crucible for growth, for example, bylongitudinally piling the appropriate number of crystal discs. As thecrucible shape for growth should be selected according to a size of afinished product, it is preferable to use a purification crucible with acorresponding shape and efficient size.

After the furnace is vacuum-exhausted, the heater 3 is electrified toheat the crucible and the material is completely melted. Then, thecrucible is descended slowly and cooled so as to grow the singlecrystal. The descending speed of the crucible depends upon the furnacestructure, but is preferably 1 mm to 5 mm per hour. The grown fluoridecrystal is then subject to the heat treatment in an anneal furnace, andthen molding into a shape of a necessary optical element.

A lens thus obtained would be applicable to projection and illuminationoptical systems suitable for excimer laser, e.g., in particular, ArFexcimer layer and F₂ excimer laser. In addition, an exposure apparatusfor photolithography may include such an optical system having a lensmade of CaF₂ crystal obtained by the inventive manufacture method, and astage for moving a substrate.

FIRST EXAMPLE

Zinc fluoride was added as scavenger and mixed with the calcium fluoridematerial by about 0.076 mol %. Then, this mixture was put in themultistage crucible having a bore of 250 mm and a depth of 150 mm in apurification furnace shown in FIG. 1 and the furnace was exhausted.Then, the crucible was heated up to 1420° C. and the material wasmelted. The crucible was then descended at a speed of 20 mm/h and cooledgradually, and the material was crystallized. The crystallized blocksize was a diameter of 250 mm and a thickness of 50 mm. Air bubbles,etc. concentrated at the top of the crystal, and thus good crystal wasobtained.

Next, the above crystal block was put in the crucible for single crystalgrowth shown in FIG. 5. The furnace was vacuum-exhausted, and thetemperature was maintained to be 1420° C. for 30 hours. Then, thecrucible for growth was descended at a speed of 1 mm/h.

Then, the grown calcium fluoride single crystal and zinc fluoride of 0.1weight % were put in a crucible in the anneal furnace. After the furnacewas vacuum-exhausted, the temperature in the crucible was heated fromthe room temperature to 900° C. at a speed of 100° C./h and maintainedat 900° C. for 20 hours.

Then, it was cooled at a speed of 6° C./h down to the room temperature.Calcium fluoride thus obtained had excellent transmittance andremarkably little deterioration.

SECOND EXAMPLE

This example is the same as the first example except for including thesteps of cutting, after the material was purified using one stage ofcrucible having a depth shown in FIG. 2, the top and bottom surfacesaccumulating impurities, and processing so that it may have a size thatfacilitates installation into the growth furnace. Calcium fluoride thusobtained had excellent transmittance and remarkably littledeterioration.

The present invention may provide calcium fluoride crystal havingexcellent optical characteristics, and its manufacture method andapparatus. An optical element made of such calcium fluoride crystal isapplicable to an optical system in an exposure apparatus for fabricatinghigh-quality devices through exposure with high resolution andthroughput.

1. A method for manufacturing fluoride crystal comprising the steps of:adding scavenger and a material to a crucible; melting the scavenger andmaterial at a temperature higher than a melting point so that a ratio ofa thickness of the fluoride crystal that has been melted to an innerdiameter of the crucible may be 0.2 or higher; and graduallycrystallizing and purifying the material.
 2. A method according to claim1, wherein the inner diameter of the crucible is 250 mm or larger.
 3. Amethod according to claim 1, wherein the crucible is divided into aplurality of parts like a multistage structure.
 4. A method according toclaim 1, further comprising the steps of: melting the purified fluoridecrystal; and growing the fluoride crystal to obtain fluoride singlecrystal.
 5. A method according to claim 1, wherein the fluoride crystalis calcium fluoride.
 6. An optical element made of fluoride crystal,said fluoride crystal being manufactured by a method including the stepsof adding scavenger and a material to a crucible, melting the scavengerand material at a temperature higher than a melting point so that aratio of a thickness of the fluoride crystal that has been melted to aninner diameter of the crucible may be 0.2 or higher, and graduallycrystallizing and purifying the material.
 7. An optical elementaccording to claim 6, wherein said optical element is a lens, adiffraction grating, an optical film or a combination thereof.
 8. Anexposure apparatus comprising an optical system including an opticalelement made of fluoride crystal, said fluoride crystal beingmanufactured by a method including the steps of adding scavenger and amaterial to a crucible, melting the scavenger and material at atemperature higher than a melting point so that a ratio of a thicknessof the fluoride crystal that has been melted to an inner diameter of thecrucible may be 0.2 or higher, and gradually crystallizing and purifyingthe material.
 9. A device fabrication method comprising the steps of:exposing an object using an exposure apparatus; and performing apredetermined process for the exposed object, wherein said exposureapparatus comprises an optical system including an optical element madeof fluoride crystal, said fluoride crystal being manufactured by amethod including the steps of adding scavenger and a material to acrucible, melting the scavenger and material at a temperature higherthan a melting point so that a ratio of a thickness of the fluoridecrystal that has been melted to an inner diameter of the crucible may be0.2 or higher, and gradually crystallizing and purifying the material.10. A device fabrication method according to claim 9, wherein saidexposing step uses ultraviolet light, far ultraviolet light, and vacuumultraviolet light.